This invention relates to the generation of infrared energy and, more particularly, to apparatus for optically pumping alkali metal vapor to cause stimulated emission of coherent infrared energy having a selected wavelength.
Coherent infrared energy may be produced by optically pumping an alkali metal vapor, such as cesium vapor for example, with visible light from a laser or the like. Pump light photons raise the cesium atoms from the 6s Rydberg energy level to a virtual level above the 5d terminal level; similarly, other d and s levels can be utilized. The subsequent transition of the excited cesium atom back to the 5d level is accompanied by emission of an infrared photon. Cascade triggering of such transitions results in an output pulse of coherent infrared radiation.
Prior devices for generating infrared by this process typically include a cylindrical heat pipe having an intermediate region in which cesium or the like is heated to vaporizing temperature and having end regions in which the vapor is cooled and condenses for recirculation to the intermediate region. Pump light is directed into the heat pipe through a window at one end, and infrared is transmitted out through another window at the opposite end. The end regions contain a cooled inert buffer gas such as neon or argon which isolates the hot vapor from the end windows to prevent condensation and loss of transparency at the windows and, in the case of certain preferred window materials, to prevent direct heat damage to the windows.
The wavelength or energy of the infrared generated by such devices is determined by the energy or wavelength of the pump light. Higher energy, shorter wavelength pump light excites a more energetic virtual Rydberg state in the vapor atoms. This results in higher energy or shorter wavelength infrared emission.
Prior efforts to utilize this effect to control or adjust the wavelength of the infrared output have not been successful except within a very narrow specific bank of wavelengths. Efficient operation, in terms of power output, has been realized only if the pump light has a wavelength close to 532 nanometers. If the pump light wavelength is shifted away from this value in prior devices of this type, efficiency falls off rapidly. This efficiency loss results from the presence of dimers or bound pairs of atoms, such as Cs.sub.2 for example, in the vapor. Through resonance effects such dimers wavelengths in the region of 532 nanometers.
Dimers dissociate or separate into individual atoms if the vapor temperature is sufficiently high. Prior efforts to resolve the above discussed problem by raising vapor temperature in the heat pipe have not achieved the desired result. It is not practical to heat the entire vapor volume to the required temperature, in part because the resulting pressure rise causes adverse effects including an increase in the rate of formation of dimers and in part because of heat damage to preferred window materials. In one prior device, a high temperature wire spiral has been situated about the axis of the heat pipe to super-heat only a localized central region of the vapor. This did not dimish the dimer concentration sufficiently to permit efficient infared generation through the possible wavelength range. Apparently dimers from the surrounding vapor volume migrate in to replace those dissociated in the immediate vicinity of the wire.
In various applications of infrared sources, such as infrared spectroscopy, air pollutant detection or absorbed monolayer analysis among other examples, it would be highly advantageous to have a greater variety of specific wavelengths available. In many cases it would be most advantageous to employ a tunable infrared source in which the output wavelength could be selectively varied throughout a broad range, preferably throughout the complete infrared spectrum. The prior devices discussed above have not provided this capability.